The present invention relates to a magnetic disc apparatus which permits the center of a magnetic disc to be precisely brought into alignment with the center of rotation.
A flexible magnetic recording medium, or floppy disc, has the advantage that it is inexpensive and easy to handle. For this reason, a magnetic disc apparatus using such a floppy disc has become very popular in recent years. In accordance with a demand for miniaturization of this magnetic disc apparatus, the floppy disc itself, which initially had a diameter of 8 inches, has been reduced to a diameter of 5 inches or even 3 inches. In order to reduce the size of the floppy disc while still providing a satisfactory storing capacity as a recording medium, it is necessary not only to increase the signal recording density per recording track, but also to increase the density of the recording tracks as well. Where the recording tracks are closer together, in order to prevent a magnetic head from disengaging from the track of the rotating floppy disc, it is necessary to have the center of the floppy disc brought exactly into alignment with the center of rotation of a disc-rotation driving means. In a floppy disc mounting mechanism of the conventional magnetic disc apparatus, however, it is impossible to obtain a highly precise alignment of the floppy disc and the disc-rotation driving means.
FIGS. 1(a) and (b) and FIG. 2 each show a floppy disc mounting mechanism of a prior art magnetic disc apparatus. In FIG. 1, a floppy disc 10 has at its center a circular bore 12. A lower holder 14 of the rotation driving means has a convex portion 14a at its upper surface, while an upper holder 16 thereof has a concave portion 16a at its lower surface. Onto this convex portion 14a, the circular bore 12 and the convex portion 16a of the upper holder 16 are fitted. Thus, the center of rotation of the floppy disc 10 is positioned and sandwiched between the holders 14 and 16. The floppy disc 10 is formed of, for example, a flexible polyester sheet having a thickness of several micrometers (.mu.m). On the other hand, the disc holder 16 is formed of rigid material. For this reason, where the bore 12 of the floppy disc 10 is deviated from the convex portion 14a at the time of fitting the disc 10 between the upper and lower holders 16 and 14, a portion of the floppy disc 10 neighboring the bore 12 is torn to bend the floppy disc 10. The precision of aligning the center of rotation of the floppy disc 10 is determined depending upon the precision of fabricating the bore 12 and the upper and lower holders 14 and 16. For this reason, the error made in positioning the rotational center of the floppy disc 10 amounts to, in most cases, from several micrometers to several hundreds of micrometers.
In FIG. 2, a rigid holder 18 is mounted on a central portion of the floppy disc 10. This holder 18 has at its center a rotational-center positioning or aligning bore 20 formed in a rectangular shape, and has toward its peripheral edge a similar-shaped disc-rotation driving bore 22. A columnar projection 26 to be fitted into the bore 20 is provided at a rotational center position of the upper surface of a spindle 24, and a columnar projection 28 to be fitted into the bore 22 is provided at a position toward the peripheral edge portion of the upper spindle surface. When the floppy disc 10 is kept in a state wherein it is mounted upon the rotation driving means, the circumferential surface of the projection 26 is kept in contact with two sides of the bore 20, whereby the center of rotation of the floppy disc 10 is positioned. Also in this case, the precision of fabricating the projection 26 of the spindle 24 and the bore 20 of the holder 18 affects the precision of aligning the center of rotation of the spindle 24 with the center of rotation of the floppy disc 10. Further, the columnar projection 28 is so urged as to be tilted in a direction such that the projection 28 goes away from the projection 26 so that the projection 26 may be brought into contact with a specified two of the sides of the bore 20. It is difficult to manufacture the spindle 24 having such a complicated structure. Since the projection 26 and the bore 20 contact each other at two points, this apparatus has the drawback that the contacting portions are liable to be worn by a centrifugal force generated when the floppy disc 10 is allowed to rotate at high speed. As mentioned above, in the prior art magnetic recording apparatus for use with a floppy disc, the precision with which the center of rotation of the floppy disc is aligned with that of the disc-rotation driving means decreases. This is a primary cause hindering the miniaturization of the floppy disc. Further, the prior art magnetic disc apparatus has the drawback that its structure is complicated and expensive to manufacture.